1. Field of the Invention
The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is concerned with an apparatus used in an automated system for inspecting a fuel assembly for envelope, channel spacing and length and also for correcting error in the inspection fixture of the apparatus.
2. Description of the Prior Art
In most nuclear reactors, the reactor core is comprised of a large number of elongated fuel assemblies. Conventional designs of these fuel assemblies include a multiplicity of fuel rods held in an organized array by grids spaced along the fuel assembly length. The grids are attached to a plurality of control rod guide thimbles. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the guide thimbles which extend above and below the opposite ends of the fuel rods. The fuel rods which contain fissile material are grouped together in a closely-spaced array within each fuel assembly and the fuel assemblies, in turn, are mounted in side-by-side closely-spaced relationship with one another between the upper and lower core plates so as to provide a neutron flux in the core sufficient to support a high rate of nuclear fission and thus the release of a large amount of energy in the form of heat.
In view of the densely-packed condition of the fuel rods and fuel assemblies in the core, the dimensional standards of envelope and length of each fuel assembly and the channel spacing between the adjacent fuel rods of each fuel assembly must be met within very close tolerances. Thus, at the completion of manufacture of each fuel assembly, quality control inspections are carried out to determine whether the fuel assembly meets the aforementioned dimensional standards. Currently, the fuel assembly quality inspection is performed at three separate stations: (1) envelope measurement; (2) channel spacing; and (3) length measurement.
At the envelope measurement station, the out-of-straightness of the fuel assembly is quantified. The sides of a fuel assembly are normally not perfectly straight. The fuel assembly commonly exhibits a slight bow and twisting. Quantifying this behavior is performed by measuring the relative position of the grids to each other and inspecting for excessive displacements. The current method of envelope measurement uses twelve LVDT sensors mounted in a configuration of three sensors per side. A set of distance measurements is taken at each grid location to signify whether the grid is located either left/right or back/front of the center of the fuel assembly. At the channel spacing station, the distance between adjacent fuel rods within a fuel assembly is checked. Currently, an operator manually pulls a strain gauge probe through a channel and a computer translates the sensor output into distance measurements. At the length measurement station, the fuel assembly length is measured using a stick micrometer. The fuel assembly is set upright on a level table and an inspector measures the distance from the table surface to the bottom edge of the top nozzle.
From the foregoing brief description of current practices, it will be readily understood that these stations are manual in nature, requiring an inspector to monitor equipment and process data. Consequently, a need has emerged to improve and automate the way in which fuel assembly inspection is carried out.